One type of RNA editing involves the conversion of adenosine residues into inosine in double-stranded RNA (dsRNA) through the action of adenosine deaminase acting on RNA (ADAR). Three ADAR gene family members (ADAR1-3) have been identified in humans and rodents. When A I RNA editing occurs within a coding sequence, synthesis of proteins not directly encoded by the genome can result, as demonstrated with transcripts of glutamate receptor (GluR) ion channels and 5-HT2C serotonin receptors. However, the most common targets for A I editing are non-coding RNAs that contain inverted repeats of repetitive elements such as Alu and LINE located within introns and 3' UTRs. The biological significance of non-coding, repetitive RNA editing is largely unknown. The overall goal of this project is to better understand the biological significance of A I RNA editing. During the past 17 years, this grant has enabled us to clone ADAR1, the first identified member of the ADAR gene family. This in turn has led to the identification and cloning of ADAR2 and ADAR3. Furthermore, we have created an ADAR1-/- mutation in mice that causes widespread apoptosis and consequent embryonic lethal phenotypes. Our results suggest that ADAR1 is critically important for survival of numerous tissues by editing a currently unknown target dsRNA(s). During the current grant support period, we found that both ADAR1 and ADAR2 edit specific adenosine residues of certain miRNA precursor dsRNAs (pri-miRNAs). Editing of pri- miRNAs results in inhibition of their processing or expression of edited mature miRNAs that silence genes different from those targeted by unedited miRNAs. Our findings revealed a previously unknown role for A I RNA editing in miRNA-mediated gene silencing. Finally, our preliminary results indicate that ADAR1 associates with certain miRNA-induced silencing complex (miRISC) member proteins. Thus, our studies strongly indicate that the RNAi and RNA editing pathways interact in mammalian cells. Therefore, in the proposed work embodies in this application, we will focus our research efforts on this RNAi and RNA editing interaction as our main thrust. Specifically, we will determine: 1) the significance of certain miRNAs that are edited during embryonic development for regulation of apoptosis; 2) the significance of A I editing of repetitive non-coding RNAs for control of endogenous siRNA (esiRNA) expression; and 3) the significance of ADAR1 interaction with the miRISC member proteins.